allow nullable $length attribute

src/Client/Request.php

@@ -112,7 +112,7 @@ class Request
         int $timeout = 30, // socket timeout, useful for offline resources
         ?int $limit = null, // content length, null for unlimited
         int $chunk = 1, // chunk size in bytes
-        int &$length = 0, // current response length, do not change without special needs
+        ?int &$length = 0, // initial response length, do not change without special needs
         ?int &$code = null, // error code for debug
         ?string &$message = null, // error message for debug
         string &$response = '' // response init, also returning by this method